Photocathode for photoemissive cells



March 21, 1967 w. HEIMANN 3,310,701

PHOTOCATHODE FOR PHOTOEMISSIVE CELLS Filed Deo. 24, 1962 United States Patent PHOTCATHODE FOR PHOTOEMISSIVE CELLS Walter Heimann, Wiesbaden-Dotzheim, Germany, assignor to Forschungslaboratorium Prof. Dr. Ing. Waiter Heimann, Wiesbaden-Dutzheim, Germany Filed Dec. 24, 1962, Ser. No. 247,481

Claims priority, application Germany, Dec. 22, 1951,

17 Claims. (Cl. 313-94) The present invention concerns a photocathode for photoemissive cells.

Conventionally such c'ells comprise a transparent bulb or vessel which is evacuated r filled with an inert gas, a photosensitive cathodeland a conductor of any suitable shape serving an anode arranged within the bulb -or vessel. When the photosensitive cathode is irradiated with luminous energy light quants are absorbed by the cathode and correspondingly electrons are released therefrom which now, under the action of an electric field existing or established between cathode and anode, travel to the latter and thusconstitute a photo-current. Since each light quant carries an amount of energy hv depending upon its wavelength and since a certain minimum photoelectric work function Le., energy is associated with the emission of electrons from the cathode, this work function depending upon the material of the cathode, a certain upper threshold wavelength is characteristically associated with each photosensitive material capable of producing a photoemissive effect, said threshold being determined by the fact that the light quants at the threshold frequency are capable to transfer when they have absorbed just enough energy for releasing one electron from the cathode surface. Thus, the above mentioned upper threshold wavelength is a characteristic constant associated with the cathode material.

In many applications of photoemissive cells this upper threshold wavelength is of great significance for determining whether a particular photocathode or photocell can be used in a particular case. Therefore efforts have been made to produce photocatho'des which are characterized by an as large as possible red threshold Wavelength because this would entail as small a work function as possible. In this respect the vbest available photosensitive cathode layersare up to now layers of cesium oxide superimposed on a layer of silver and covered with another layer of monoatomic cesium. The upper threshold wavelength of this cathode material is located'at about 1.2M. In this case the partial layer consisting of CszO is to be considered as a semiconductor of p-type conductivity.

It is one object of this invention to provide for a photocathode which is characterized by an upper threshold wavelength which is located still further down in the red portion of thespectrum so that hereby the photoelectric work function is still further reduced as compared with conventional photocathodes.

It is a further object of the invention to provide for a photocathode of the type set forth which is comparatively easy to manufacture and entirely reliable in operation.

With above objects in View the invention includes a photocathode for photoemissive cells comprising, in combination, a first layer of semiconductor material; and at least one second layer of semiconductor material superimposed on said first layer, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effect uponwgbeing irradiated with luminous energy, the conductivity types and carrier concentrations pared with conventional one-layer photocathodes.

liltijil Patentes Maf. 21, las? of said semiconductor materials of said layers, respectively, being so chosen in relation to each other that the polarity of the appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoelectric work function of said layer having photoemissive effect.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing, in which:

FIG. l is an axial cross section of a photoemissive cell comprising one embodiment of a photocathode according to the invention; and

FIG. 2 is a similar cross section of a photoemissive .cell comprising another embodiment of the invention.

Before describing the details of the illustrated embodiments of the invention it may be stated that fundamentally photocathodes according to the invention are essentially constructed as a combination of a conventional photosensitive pn-type semiconductor layer with a second photosensitive semiconductor layer of known type and having an external photoelectric effect eg., a photocathode layer composed of Ag-{Cs2O-j-Cs. In other words, the photocathode according to the invention constitutes a combination of a junction layer having an internal photoeffect with a superimposed photoemissive layer, the conductivity types of the semiconductor materials of said layers, respectively, being such that the polarity ofy the appearing at the blocking region existing and normally developing between said layers upon absorption of light is opposite to that of the photoelectric Work function of the photoemissive layer so that in this manner the work function is substantially reduced as com- It is not necessary that the semiconductor materials of the first and second layer differ from each other, it is sufficient if the carrier concentrations thereof differ from each 'other in such a manner that the above mentioned effect is obtained.

Referring now to the drawing, in FIGS. l and 2 photoemissive cells are illustrated comprising a bulb or' vessel 1 made at least partially of transparent material, preferably evacuated and provided with input leads 2 and an anode of any suitable shape and provided with an output lead 5. The material of the leads 2 and 5' must be compatible with the transparent material of the bulb 1. If the latter is made of glass then suitable lead materials which can be welded into the glass are metals as for instance platinum, copper, ferrochromium and the like as is well known.

In the embodiment according to FIG. l the photocathode is composed of a first semiconductor layer 3 having an internal photoeffect and applied to one wall of the bulb 1 so as :to have intimate conductive contact with the input leads 2. Superimposed on the first layer 3 is a second semiconductor layer 4 having photoemissive ability and being in Contact with the first layer 3 along the bound- `ary surface 3a. Upon irradiation of this composite photocathode with luminous energy electrons 6 will be emitted and caused to travel toward the anode 5 provided that an operating potential is applied to the cell.

It will be understood that only for the sake of clarity of the illustration the thickness of the layers 3 and 4 is greatly exaggerated.

If it is desired to apply the irradiating luminous energy in the direction of the arrows 7 then -it would be advantageous and advisable to use for both layers 3 and 4 a thickness which is small enough to render both these semiconductor layers transparent. However there may be applications or uses of a photocell as illustrated in which it would be suiiicient to make only one or the other of the layers 3 and 4 transparent. The transparency of these layers depends only on the layer thickness which may range between 0.01 and 10a.

For instance, the photoemissive layer 4 may consist of the well known p-type construction Ag-i-CszO, while the adjacent semiconductor layer 3 may have n-type conductivity, e.g. being made of silicon containing P, As, Sb or similar impurities as is well known. In this case, while the photocell is not irradiated, and while no outside potential is applied to the cell, a blocking layer having a certain insulating effect is formed as indicated at 8 due to diffusion of electrons from the n-type layer into the p-type layer, and, on the other hand, ofthe holes from the p-type layer into the n-type layer.

Assuming, only the layer 4 alone were irradiated, then the resulting emission of electrons leaving this layer would have only the effect that the hole concentration therein is increased whereby the diffusion potential would be increased. However, the arrangement of the ltwo layers 3 and 4 is such that the luminous energy irradiating the layer 4 simultaneously also irradiates the semiconductor layer 3 which has an internal photoeffect. This causes the formation of charge carriers inthe area of the blocking layer, these carriers being electrons and holes depending upon the different areas of conductivity therein, so that these carriers travel away from their characteristic zones and create a s-tationary condition whereby a photodevelops the potential and polarity whereof is opposed to that of the photo electric work function of th-e photoemissive layer. Consequently this work function is hereby reducedand as a result the upper threshold wavelength is increased. Thus the desired result of the invention is obtained.

lFIG. 2 differs from FG. l only in that an insulating layer 3a is provided between the semiconductor layers 3 and 4 in order to prevent undesirable chemical reactions between the materials of the semiconductor layers 3 and 4 prov-ided that the latter are of different type. Suitable materials for the insulating layer 3a are substances which are incapable of chemically reacting with either one of the semiconductor materials of the layers 3 and 4. Suitable materials are metal oxides as for instances SiO2 or GeO2. The thickness of this separating or insulating layer should be in the range between 0.0M and 10u, and the material in this layer may be in the form of a powder, a gel or a paste.

While in the above described examples mainly only two layers 3 and 4 are combined Ito form a photocathode, the number of layers may be increased if so desired provided that the relation between consecutive layers is as described above.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of a photocathode for photoemissive cells differing from the types described above.

While the invention has been illustrated and described as embodied in a photocathode for photoemissive cells comprising a plurality of layers of semiconductor materials, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, ythe foregoing will so fully reveal the gist of the present invention that others can *by applying current knowledge readily adapt it for variout applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are infit) tended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A photocathode for photoemissive cells, comprising, in combination, a first layer of semiconductor material; and at least one second layer of semiconductor material superimposed on said rst layer, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effect upon being irradiated with luminous energy, the conductivity types and carrier concentrations of said semiconductor materials of said layers. respectively, being so chosen in relation to each other that the polarity of the E.M.F. appearing at the blocking region existing between said first and second layers upon absorption of said luminous'energy is such as to be opposite to that of the photo? electric work function of said layer having photoemissive Y 2. A photocathode for photoemissive cells, comprising, in combination, a first layer of semiconductor material having a thickness which renders said first layer transparent to a predetermined degree; and at least one second layer of semiconductor material superimposed on said first layer, at least one of said layersof semiconductor material containing a p11-junction having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effect upon being irradiated with luminous energy, the conductivity types of carrier concentrations of said semiconductor materials of said layers, respectively, being so chosen in relation to each other that the polarity of the appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoelectric work function of said layer having photoemissive effect. v y' 3. A photocathode for photoemissive cells, comprising',

in combination, a first'layer of semiconductor materiall having a thickness which renders said lfirst layer transparent to a predetermined degree; and at least one second layer of semiconductor material superimposed on said first layer and having a thickness which renders said second layer transparent to a predetermined degree, at least one of said layers of semiconductor material con'- taining a pn-junction having a photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effect upon being irradiated with luminous energy, the conductivity types and carrier concentrations of said semiconductor materials of said layers, respectively, being so chosen in relation to each other that the polarity of appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be .y opposite to that of the photoelectric work function of said layer having photoemissive effect.

4. A photocathode for photoemissive cells, comprising, in combination, a rst layer of semiconductor material; and at least one second layer of semiconductor material superimposed on said first layer, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effect upon being irradiated with luminous energy, the conductivity types of said semiconductor materials of said layers being equal and their carrier concentrations, respectively, being so chosen in relation to each other that the polarity of the appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such having a thickness Which renders said fist layer transparent to a predetermined degree; and at least one second layer of semiconductor material superimposed on said ,first layer, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effect upon being irradiated with luminous energy, the conductivity types of said semiconductor materials of said layers being equal and their carrier concentrations, respectively, being so chosen in relation to each other that the polarity of the appearing at the blocking region existing between said first and secon-d layers upon absorption of said lumilnous energy is such as to be opposite to that of the photoelectric Work function of said layer having photoemissive effect.

6. A photocathode for photoemissive cells,-comprising, .in combination, a first layer of semiconductor material having a thickness which renders said first layer transparent to a predetermined degree; and at least one second layer of semiconductor material superimposed on said first layer and having a thickness which renders said secon-d layer transparent to a predetermined degree, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effect upon vbeing irradiated with luminous energy, the conductivity of said semiconductor materials of said layers being equal and their carrier concentrations, respectively, being so chosen in relation to each other that the polarity of the appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoelectric work function of said layer having photoemissive effect.

7. A phot-ocathode for photoemissive cells, comprising, in combination, a first layer of semiconductor material; and at least one second layer of semiconductor material superimposed on said first layer, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect,` and at least one other one of said layers of semiconductor mlaterial having photoemissive effect upon being irradiated with luminous 6 at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effect upon being irradiated with luminous energy, the conductivity types of said semiconductor materials of said layers, respectively, being energy, the conductivity types of said semiconductor materials of said layers, respectively,being different from each other and so chosen in relation .to each other that the polarity of the appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoelectric Work function of said layer having photoemissive effect.

8. A photocathode'for photoemissive cells, comprising, in combination, a first layer of semiconductormaterial havin-g a thickness which renders said first layer transparent to a predetermined degree; at least one second layer of semiconductor material superimposed on said first layer, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effect upon being irradiated with luminous energy, the conductivity types of said semi-conductor materials of said layers, respectively, being different from each other and so chosen in relation to each other that the polarity of the E.M.F. appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoelectric work function of said lay-er having photoemissive effect.

9. A ,photocathode for photoemissive cells, comprising, in combination, a first ylayer of semiconductor material having a thickness which renders said first Alayer transparent to a predetermined degree; and at least one second layer of semiconductor material superimposed on said first layer and having a thickness which renders said second layer transparent to a predetermined degree,

different from each other and so chosen in relation to each other that the polarity of the appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoel-ectric `Work function of said layer Ihaving photoemissive effect.

10. A photocathode for photoemissive cells, comprising, in combination, a first layer of semiconductor material; and at least one second layer of semiconductor material superimposed on said first layer, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one vof said layers of semiconductor material having photoemissive effect upon being irradiated with luminous energy, the conductivity types and carrier concentrations of said semiconductor materials of said layers, respectively, being so chosen in -relation to each other that the `polarity of the appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoelectric work function of said layer having photoemissive effect; and a thin separating layer of material incapable of chemically reacting with said semiconducting materials of said first and second layers being interposed between the latter for preventing chemical inter-reactions between the same.

11. A photocathode as claimed in claim 10, wherein said separating layer is composed of a finely distributed meta-l oxide. l

12. A pho-tocathode for photoemissive cells, comprising, in combination, a first layer of semiconductor material; and at least one second layer of semiconductor material superimposed on said first layer, at leastone of said layers of semiconductor material containing a pnjunctilon having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive effe-ct upon being irradiated with luminous energy, the conductivity types of said semiconductor materials of said layers being equal and their carrier concentrations, respectively, being so chosen in relation tto each other that the polarity of the appearing at the blocking region existing between said first 'and second layers upon absorption of saidluminous energy is such as to be opposite to that ofthe pho-toelectric wo-rk function of said layer having .photoemissive effect; and a thin separating layer of material incapable of chemicalll'y reacting with said semi-condu-ctor materials of said first and second layers being interposed between the latter for preventing chemical inter-reactions between the same.

13. A photocathvode for photoemissive cells, `comprising, in combination, a first layer of semiconductor material; and at least one second layer `of semiconductor material superimposed on said first layer, at leastone of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one of Vsaid layers of semiconductor Imaterial having photoemissive effect upon being irradiated with luminous energy, the conductivity types of said semiconductor materials of said layers, respectively, being different from each other and so chosen in relation to each other that the polarity of the appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoelectric work function of said layer having photoemissive effect; and a thin separating layer of material incapable of chemically reacting with said semic-onducting materials of said -first and second layers being interposed between the latter fior preventing chemical inter-reactions between the same.

14. A photocathode for photoemissive cells, comprising, in combination, a first layer of semiconductor material having a thickness which renders said first layer transparent to a predetermined degree; and at least one second l-ayer of semiconductor material superimposed on said first layer, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effeet, and at least one other one o-f said layers of semiconductor material having photoemissive effect upon being irradiated wit-h luminous energy, the conductivity types of said semiconductor materials of said layers being equal and their carrier concentrations, respectively, being so chosen in rela-tion to each other that the polarity of the appearing at the blocking region existing between said iirst and second layers upon absorption of said luminous energy in such as to be opposite to that of the photoelectric work function of said layer having photoemissive effect, and a thin separating layer of material incapable of chemically reacting with said semiconductor materials of said first and second layers being interposed between the latter for preventing chemical inter-reactions between the same.

15. A photocathode for photoemissive cells, comprising, in combination, a first layer of semiconductor material having a thickness which renders said first layer transparent to a predetermined degree; and at least one second layer of semiconductor material superimposed on said iirst layer and having a thickness which renders said second layer transparent to a predetermined degree, at least lone of said layers of semiconductor material containing a pn-junction having vphotovoltaic eliect, and at least one other one of said layers of semiconductor material having photoemissive effect upon being irradiated with luminous energy, the conductivity types of said semiconductor materials of said layers being equal and their carrier concentrations, respectively, being so chosen in relation to each other that the polarity of the E.l\/l.-F. appearing at the blocking region existing between said iirst and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoelectric work function of said layer having photoemissive effect; and a thin separating layer of material incapable of chemically reacting with said semiconductor materials of said rst and se'cond layers being interposed between the latter for preventing chemical inter-reactions between the same.

16. A photocathode for photoemissive cells, comprising, in combination, a first laye-r of semiconductor material having a thickness which renders said irst layer transparent to a predetermined degree; and at least one second layer of semiconductor material superimposed on said iirst layer, at least one of said layers of semiconductor material containing a pn-junction having photovoltaic effect, and at least one other one o'f said layers of semiconductor material having photoemissive etfect upon being irradiated with luminous energy, the conductivity types of said semiconductor materials of said layers, respectively, being different from each other and so chosen lin relation to each other that the polarity of the appearing at the blocking region existing between said first and second layers upon absorption of said luminous energy is such as to be opposite to that of the photoelectric work function of said layer having photoemissive effect; and a thin separating layer of material incapable of chemically reacting with said sem'iconducting materials of said first and second layers being interposed between the latter for preventing chemical inter-reactions between the same.

17. A photocathode for photoemissive cells, comprising, in combination, a irst layer of semiconductor material having a thickness which renders said iirst layer transparent to a predetermined degree; and at least one second layer of semic-onductor material superimposed on said first layer and having a thickness which renders said second layer transparent to a predetermined degree, at least one of said layers ofsemiconductor material containing a pn-junction having photovoltaic effect, and at least one other one of said layers of semiconductor material having photoemissive eiTect upon being irradiated with luminous energy, the conductivity types of said semiconductor materials ofsaid layers, respectively, being different from each other and so chosen in relation to each other that the polarity of the appearing at the blocking regiony existing between said lirst and second layers upon absorption' of sa-id luminous energy is such as to be opposite to that of the photoelectric work vfunction of said layer having photoemissive effect; and a thin separating layer of material incapable of chemically reacting with said semiconducting `materials ofv said first and second layers being interposed between the latter for preventing chemical inter-reactions between the same.

References' Cited by the Examiner UNITED STATES PATENTS 2,118,186 5/19'38 isamsworrh` 313-95 2,766,144 10/1956 Lidow 317-234 l2,873,302 2/1959 Ratner 317-235 3,005,107 10/1961 Weinstein 307-885 3,049,622 8/1962 Ahlstrom et al. 317-234 FOREIGN PATENTS 965,706 7/1957 Germany.

JOHN W, HUCKERT, Primary Examiner'.

A. JAMES, Assistant Examiner. 

1. A PHOTOCATHODE FOR PHOTOEMISSIVE CELLS, COMPRISING, IN COMBINTON, A FIRST LAYER OF SEMICONDUCTOR MATERIAL; AND AT LEAST ONE SECOND LAYER OF SEMICONDUCTOR MATERIAL SUPERIMPOSED ON SAID FIRST LAYER, AT LEAST ONE OF SAID LAYERS OF SEMICONDUCTOR MATERIAL CONTAINING A PN-JUNCTION HAVING PHOTOVOLTAIC EFFECT, AND AT LEAST ONE OTHER ONE OF SAID LAYERS OF SEMICONDUCTOR MATERIAL HAVING PHOTOEMISSIVE EFFECT UPON BEING IRRADIATED ITH LUMINOUS ENERGY, THE CONDUCTIVITY TYPES AND CARRIER CONCENTRATIONS OF SAID SEMICONDUCTOR MATERIALS OF SAID LAYERS, RESPECTIVELY, BEING SO CHOSEN IN RELATION TO EACH OTHER THAT THE POLARITY OF THE E.M.F. APPEARING AT THE BLOCKING REGION EXISTING BETWEEN SAID FIRST AND SECOND LAYERS UPON ABSORPTION OF SAID LUMINOUS ENERGY IS SUCH AS TO BE OPPOSITE TO THAT OF THE PHOTOELECTRIC WORK FUNCTION OF SAID LAYER HAVING PHOTOEMISSIVE EFFECT. 